Light and energy blocking and color correction have typically been imparted to optical products such as solar control films by use of organic dyes. More particularly, current commercial practice for producing dyed film from polyester includes swelling of the molecular structure of the polyester in baths of hot organic solvent such as ethylene glycol during the dyeing process, as swelled polyester (particularly PET) films are capable of absorbing organic dyes. These films and their manufacturing processes suffer many drawbacks. For example, only a limited number of organic dyes are soluble and stable in the hot solvent swelling media used in the dyeing process and many of those are subject to degradation by high energy radiation (sub-400 nm wavelength) to which the substrate is subjected when used in window film applications, thereby shortening the useful lifetime of the product. Further, the dye typically permeates the entire thickness of the film such that there is no way to separate the filtration or blocking function of the film from its visible reflection, that is, from its external appearance.
To address some of these drawbacks, some film manufacturers have transitioned to using a pigmented layer on the surface of a base polymeric film for tinting a polymeric film. For example, U.S. Published Application number 2005/0019550A1 describes color-stable, pigmented optical bodies comprising a single or multiple layer core having at least one layer of an oriented thermoplastic polymer material having dispersed within it a particulate pigment. Such products can suffer a myriad of processing and performance drawbacks. For example, layers of this type are typically applied as thin films and require a relatively high pigment concentration to achieve a desired tint level, particularly in automotive window films with a relatively high desired level of darkening. These high pigment concentrations are difficult to uniformly disperse within the thin layer. And again, it may be difficult to separate the desired blocking properties of the product from its external appearance or reflection since the pigment is dispersed throughout the thickness of the thermoplastic polymer material.
More recently, layer-by-layer films have been developed to provide similar functionalities. Processes of making these films take advantage of charge-charge, hydrogen bonding, or other complementary interactions to assemble successive layers. This requires the use of solvents, typically water, to ionize molecules or support hydrogen donation and acceptance in the deposition solutions.
For example, U.S. Pat. Nos. 9,453,949 and 9,891,357, assigned to the assignee of the present application, the disclosures of which are incorporated herein by reference, disclose layer-by-layer composite films that include electromagnetic energy-absorbing insoluble particles that may be selected to provide pigmentation, UV-absorption, and/or IR-absorption properties. Likewise, U.S. Pat. No. 9,395,475, the disclosure of which is incorporated herein by reference, discloses layer-by-layer integrated stacks that serve as optical filters.
Optical products are known that can selectively block certain wavelengths of light, for example blue light that is believed to contribute to macular degeneration, cataracts, and the like. For example, shooting glasses having lenses containing yellow dyes are known to block blue light, but have a clearly discernible yellow tint to both the user and the observer. U.S. Pat. No. 8,882,267 discloses ophthalmic and nonophthalmic systems that are said to provide an average transmission of 80% or better transmission of visible light, to inhibit selective wavelengths of blue light, to allow for the wearer's proper color vision performance, and to provide a mostly color neutral appearance to an observer looking at the wearer wearing such a lens. The systems are said to be color balanced such that the yellow or amber color, or other unwanted effect of blue blocking is reduced, offset, neutralized or otherwise compensated for, so as to produce a cosmetically acceptable result, without at the same time reducing the effectiveness of the blue blocking.
A continuing need exists in the art for electromagnetic energy-absorbing optical products, and especially window films, that selectively filter bands of energy from transmitted visible light, producing a distinct transmitted color, while remaining relatively neutral in reflected color.